1. Field of the Invention
The present invention generally relates to communications technology and, more particularly, to a Digital Video Broadcasting Cable (DVB-C) system based on Orthogonal Frequency Division Multiplexing (OFDM) and a method for processing a reserved tone therein.
2. Description of the Related Art
Current communication systems offer various enhanced communication services with high-rate data transmissions and improved Quality of Service (QoS). Some of these current communication systems are based on OFDM.
OFDM is a multi-carrier technique that allows data transmission through a number of subcarriers disposed to maintain orthogonality amongst the subcarriers. OFDM typically has a higher efficiency of frequency use than various other multiplexing schemes and is tolerant of multi-path fading. Therefore, OFDM has been widely adopted in standards of mass communication systems such as the Digital Audio Broadcasting (DAB) system and the Digital Video Broadcasting—Terrestrial (DVB-T) system. The DVB-T system uses a single channel band to offer a broadcasting service.
However, a traditional Digital Video Broadcasting—Cable (DVB-C) system was based on a single-carrier technique that allows data transmission through a single subcarrier. Contrary to the DVB-T system, the DVB-C system uses a number of channel bands to offer a broadcasting service, and thus DVB-C has a higher efficiency of frequency use. In the DVB-C system, channel bands are organized in a simple link structure as shown in FIG. 1. Therefore, as the number of channel bands increases, guard bands located at both sides of each individual channel band also increase in number. For example, if four channel bands are employed in the DVB-C system, a bandwidth actually available for a broadcasting service is limited to the sum of four channel bands excluding the eight guard bands.
In view of the above-described bandwidth limitation, a DVB-C second generation (DVB-C2) system not only employs at least one channel band to offer a broadcasting service, but also is based on OFDM. In the DVB-C2 system, channel bands combine into a multi-channel bundle as shown in FIG. 2. Therefore, even when the number of channel bands increases, the number of guard bands does not change. For example, when four channel bands are used according to the DVB-C2 system illustration of FIG. 2, a bandwidth actually available for a broadcasting service excludes only two guard bands. Therefore, the DVB-C2 system may have better frequency efficiency than the traditional DVB-C system or the DVB-T system.
However, the DVB-C2 system may have shortcomings in that a Peak to Average Power Ratio (PAPR) is relatively higher than the other systems, due to the adoption of OFDM. Specifically, since the amplitude of a broadcast signal based on OFDM is equal to the sum of the amplitudes of subcarriers, such a broadcast signal may be inherently subjected to a significant fluctuation in amplitude. Additionally, this amplitude fluctuation may become significant when the phases of subcarriers coincide with each other. Therefore, the performance of the DVB-C2 system may be deteriorated by the amplitude fluctuations.